1. Field of the Invention
The invention pertains to improved electrooptical devices for switching pluralities of unpolarized or polarized optical signals between input and output multimode singlestrand fiber optical guides and more particularly relates to novel electrooptical switch elements and matrices employing electric field-effect liquid crystal compositions and adaptable for the construction of large scale, integrated multiple switch arrays.
2. Description of the Prior Art
It has been proposed to use nematic liquid crystal materials in unit optical switches; but generally, the single switch configurations proposed have inherent geometric and other properties making them not particularly useful for combination in simple optical multiple switch matrices. Nonetheless, liquid crystal materials are characterized by large changes in optical index of refraction for only modest changes in the electric field at relatively low electric field magnitudes and have other significant advantages, as are discussed widely in the literature and, for instance, in the U.S. patent application Ser. No. 947,983, now U.S. Pat. No. 4,201,442, by D. H. McMahon and R. A. Soref and for a "Liquid Crystal Switching Coupler Matrix", filed Oct. 2, 1978 and assigned to Sperry Corporation.
The faults of the prior art are further discussed in the R. A. Soref U.S. patent application Ser. No. 13,095, now abandoned, for "Liquid Crystal Switching Matrices", filed Feb. 21, 1979 and assigned to Sperry Corporation. The Soref invention provides novel electrooptical matrix switches for selectively switching pluralities of optical signals between collimating output fiber light guides of the multimode, single strand kind. Switching is accomplished by selective electric field excitation of positive or of negative anisotropy liquid crystal media. The geometry of the multiple switch structure may be regarded as a hybrid between the modular and non-modular; in certain forms of the invention, sets of similar trapezoidal prismatic glass parts are economically employed and, between those glass prisms, thin planar liquid crystal layer cells are defined which include a plurality of active electrooptic regions. The complete configuration may have one or more degrees of geometric symmetry. The assembly of prisms and liquid crystal layers may cooperate, for example, in directing light signals from three inputs to selected ones of three outputs, while certain of the prisms cooperate with light absorbers for removing unwanted scattered light.
Also of interest is the related R. A. Soref U.S. patent application Ser. No. 97,294, now U.S. Pat. No. 4,278,327, for "Improved Liquid Crystal Matrices", filed Nov. 26, 1979 and assigned to Sperry Corporation. The invention again relates to improved electrooptic devices for switching unpolarized or polarized optical signals between input and output multimode singlestrand optical wave guides, unit switches employing electric field effect liquid-crystal compositions and adaptable to the construction of large scale, integrated multiple switch arrays. Switching is accomplished by the selective electric field excitation of liquid crystal media confined in layers between prismatic glass elements supporting a plurality of cooperating mirror and transparent electrode systems. Switching is accomplished by means provided for rendering operation efficiently independent of the particular polarization of the incoming light. Some forms of the invention further benefit because the selected liquid crystal material, in cooperation with the selected disposal of mirrors and optically active regions defined between active transparent electrodes, makes it possible to use only one type of liquid crystal material throughout each liquid crystal layer, eliminating the need for internal separating barriers and for an inert index-matching liquid employing such barriers to separate the liquid from the liquid crystal material.
While electrooptical switches of the foregoing types are attractive for use in many applications, certain of these devices require large pluralities of high index ground glass prisms, as well as of intervening liquid crystal cells, leading to relatively high manufacturing cost. While utility in matrices is demonstrated where the number of switching possibilities is low, excessive cost begins to result when the number of switching elements is considerably expanded. Also, certain of the prior art devices suffer from cross-talk levels that are sometimes higher than the desirable theoretically predicted values and that are not equal in both the excited and unexcited states of each switching element. Accordingly, it is the primary objective of the present invention to supply efficient multimode optical matrix switches that route light from multiple fiber inputs to multiple fiber outputs in response to electrical command signals, reciprocal compact matrix switches which may accommodate many inputs and many outputs with further minimized cross-talk and optical insertion losses.